Automatic humidity step control thermal analysis apparatus

ABSTRACT

Humidity dependency of a physical property of the sample is evaluated while introducing the saturated vapor corresponding to the wet-bulb temperature into the sample chamber in which the temperature is controlled corresponding to the dry-bulb temperature of the wet-and dry-bulb type hygrometer. At this point, by changing the temperature corresponding to the wet-bulb temperature at the timing when physical property signals of the sample become stable, the different saturated vapor is introduced into the sample chamber, then the humidity of the sample chamber is automatically changed in a stepwise manner. The humidity of the sample chamber is always changed after the physical properties of the sample becomes stable, so that the knowledge about the time required for the sample to reach the equilibrium state is not necessary, and humidity dependency of physical properties of the sample can be measured accurately.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a novel improvement on a thermal analysis apparatus for analyzing changes of physical properties of a material with temperature, where the effect of the humidity in the periphery of the sample on the physical properties of the material can be evaluated.

[0002] Thermal analysis is a powerful method for studying how the physical properties of a material change with temperature. As examples of a typical measurement device, Differential Thermal Analyzer (DTA), Differential Scanning Calorimeter (DSC), Thermogravimetric Analyzer (TG), Thermomechanical Analyzer (TMA), Dynamic Mechanical Analyzer (DMA), etc., are widely used. An improvement on thermal analysis apparatus to provide humidity control function is disclosed in Japanese Patent laid open No. Tokkaihei. 8-145918.

[0003] As an automatic moisture absorption equilibrium measurement device, a combined microbalance and humidification device is on the market.

[0004] In the aforementioned related arts, a thermal analysis apparatus with humidity control function described in Patent Laid open No. Tokkaihei. 8-145918, the program control is performed not only for the temperature but also for the humidity in the periphery of the sample. The humidity scanning measurement enabled by performing program control for humidity provides a great advantage. On the other hand, compared to the case of temperature equilibrium, the time required for the physical property of the sample at a certain humidity to reach equilibrium is extraordinary long. There is the problem that evaluation of the humidity dependency of the physical property of the samples inaccurate unless the humidity scanning speed is set very low. Moreover, since the time required for the sample to reach humidity equilibrium depends on shape and properties of the sample, and there is the problem that the humidity program cannot be built adequately without obtaining knowledge about the hygroscopicity of the sample in advance.

[0005] On the other hand, an automatic moisture absorption equilibrium measurement device obtains desired relative humidity by mixing dry nitrogen and saturated vapor at a certain ratio, and comprises a precision gas flow rate control device, a vapor generating device, a mixing device, drainage device, and microbalance, etc. The disadvantages are that it is expensive because of its complicated device composition, and it also lacks versatility as it cannot be used with thermal analysis apparatus.

[0006] To measure the sample absorbed moisture at a certain humidity and reach the equilibrium state with the thermal analysis apparatus of the related art, during sampling and introduction of the sample, the device is exposed to atmosphere in which the humidity is notcontrolled, therefore, there is a problem that the system deviates from the equilibrium state before the measurement starts.

SUMMARY OF THE INVENTION

[0007] To resolve the above problems, according to the present invention, a hot water chamber, capable of temperature step control, is utilized for generating saturated vapor at the appropriate temperature, a chamber capable of changing the temperature and humidity of the sample, a heat insulation pipe for transmitting vapor from the hot water chamber to the sample chamber while preventing from dew condensation, a detector for measuring physical properties of the sample, and a signal stability determination circuit for generating a trigger signal in case that the rate of change of the physical property signal from the detector becomes slower than a specified reference value. In this case, the humidity of the sample chamber is determined according to the ratio of the saturated vapor pressures at the temperature of the hot water chamber and at the temperature of the sample chamber respectively. The water temperature of the hot water chamber is changed corresponding to the trigger signal in a stepwise manner, accordingly, the humidity of the sample chamber is also changed in a stepwise manner. As a result, humidity equilibrium of sample and the atmosphere in the periphery of the sample is confirmed based on the physical property of the sample, and humidity condition is then automatically scanned in a stepwise manner.

[0008] And, if the sample, which previously absorbed moisture at a certain humidity and reached the equilibrium state, is caused to deviate from its state of equilibrium temporarily, the signal stability determination circuit determines the change of the physical property signal by the controlling humidity of the sample chamber and confirms whether or not it returns to its state of equilibrium with the trigger signal at the measurement start.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a block diagram with cross section through a part showing an embodiment of the present invention.

[0010]FIG. 2 is a schematic time chart of the signal obtained in the embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of the present invention will be described in the following. A device for thermogravimetry measurement with application of the present invention is described in FIG. 1. 1 in FIG. 1 is a sample to be measured. A heat-retaining furnace 2 made of stainless steel with a cylindrical inside cavity is arranged to wrap around sample 1. One end of the heat-retaining furnace 2 is sealed composing a bottom surface, and another end is open. A constant temperature circulator bath 4 capable of temperature setting is connected to heat-retaining furnace 2 via hose 3, 3. Liquid medium, for example, water, to a constant temperature circulator bath 4, is circulated to and from the cavity within the heat-retaining furnace 2 via hose 3, to make the temperature of the whole of the heat-retaining furnace 2 close to the set temperature of the constant temperature circulator bath 4.

[0012] The heat insulation pipe 5 penetrates through the bottom surface of the heat-retaining furnace 2. The other end of the heat insulation pipe 5 is introduced into the inside of the glass bottle 8. The heat insulation pipe temperature control device 7 is connected to the heater 6 embedded in the heat insulation pipe 5. The glass bottle 8 is filled with distilled water, approximately seven-tenths, and plugged with the rubber stopper 10, which is penetrated by the bubbling gas introduction pipe 9 and also by a teflon tube core composing the heat insulation pipe 5. The glass bottle 8 is fixed in the water within the constant temperature bath 11, the temperature of which is capable of being preset, with almost the whole glass bottle submersed in the water, so that the temperature inside the glass bottle 8 is close to the set temperature of the constant temperature bath 11. One of the end of the bubbling gas introduction pipe 9 is connected to the flow meter12.

[0013] A sample container containing the sample 1 is mounted on the sample holder 13. Change of weight and temperature of the sample 1 is measured at the balance unit 15 provided inside the balance case 16 after passing through the balance beam 14 with the thermocouple embedded. The flow meter 18 is connected to a part of the balance case 16 via the purge gas introduction tube 17. The thermogravimetry measurement device shown in FIG. 1 is of a horizontal type with balance beam 14 provided in a horizontal manner. This also can be applied to the thermal gravity measurement device, for example, an above-balance type or a below-balance type.

[0014] The heat-retaining furnace 2 is provided with the gas outlet 27, and with the humidity sensor 19 in its inside. The humidity of the atmosphere in the periphery of the sample 1 is measured at the humidity measuring circuit 20 connected to the humidity sensor 19. On the other hand, changes of weight and temperature of sample 1 are measured at the TG measuring circuit 21connected to the balance unit 15. The thermal stability determination circuit 22 is connected to the humidity measuring circuit 20 and the TG measuring circuit 1. The temperature and humidity condition input device 23 is connected to the signal stability determination circuit 22. The signal stability determination circuit 22 is also connected to the temperature and humidity target signal generating device 24. The circulator bath temperature setting device 25 and the constant temperature bath temperature setting device 26 are also connected to the temperature and humidity target signal generating device 24. The constant temperature circulator bath 4 is connected to the circulator bath temperature setting device 25, and the constant temperature bath 11 is connected to the constant temperature bath temperature setting device 26.

[0015] Next the operation of the apparatus shown in FIG. 1 is described.

[0016] Prior to measurement, the operator sets the flow rate of purge gas at 200 ml/min with the flow meter 18 to protect the balance unit from dew condensation, then introduces the purge gas into the balance case 16 via purge gas introduction tube 17. The flow rate of the flow meter 12 is set at 200 ml/min, and dry air or nitrogen gas is allowed to flow. The air or nitrogen gas flowing from the flow meter12 is transmitted into the glass bottle 8 via bubbling gas introduction pipe 9 which is introduced in the vicinity of the bottom surface in the water within the glass bottle 8, and is introduced, as gas including the vapor at saturated vapor pressure corresponding to the water temperature, in the periphery of the sample 1 within the heat-retaining furnace 2 via the heat insulation pipe 5. The open end of the heat-retaining furnace 2 is in close contact with the balance case 16, and the purge gas and the saturated vapor are both exhausted to the outside via the gas outlet 27.

[0017] Next, the heat-retaining furnace 2 is moved, and the sample 1 to be measured is placed in the container and then in the sample holder 13, and the heat-retaining furnace 2 is replaced.(the moving means for heat-retaining furnace is not shown). The temperature and humidity range over which measurements are to be performed, for example, 50□, relative humidity 30%˜70%, etc, are input at the temperature and humidity condition set device 23, and then measurement is started.

[0018] When the measurement is started, using the input signal of the temperature and humidity condition set device 23, the target signal of the temperature and humidity is immediately transmitted from the temperature and humidity target signal generating device 24 to the circulator bath temperature setting device 25 and the constant temperature bath temperature setting device 26. At the circulator bath temperature setting device 25, the target signal for temperature is extracted and sent to the constant temperature circulator bath 4. At the constant temperature circulator bath 4, temperature control is started, setting its target value at the temperature signal from the circulator bath temperature setting device 25. As a result of thermal transmission caused by the water circulating inside, after a short time the temperature of the heat-retaining furnace 2 connected to the constant temperature circulator bath 4 via the hose 3 will be equal to the temperature of the constant temperature circulator bath 4. Also, the temperature of the sample 1 gets close to the temperature of the heat-retaining furnace 2. As a result, the temperature of the sample 1 gets close to the temperature of the constant temperature circulator bath 4, namely, the target temperature outputted from the circulator bath temperature setting device 25. The temperature of the sample 1 is detected by the thermocouple(not shown) welded on the bottom surface of the sample holder 13, and converted into measurement temperature signal at the TG measuring circuit 2 after transmitted through the balance beam 14, and the balance system 15.

[0019] On the other hand, at the constant temperature bath temperature setting device 26, the target temperature of the constant temperature bath 11 is calculated as follows based on the target temperature and humidity condition.

[0020] That is, if the temperature of the constant temperature bath 11 is taken to be Tw, the temperature inside the glass bottle 8 will be Tw. If the saturated vapor pressure at the temperature T is taken to be P(T), the vapor pressure of the gas introduced in the periphery of the sample via the heat insulation pipe 5 is given by P(Tw). The temperature of the heat insulation pipe 5 is kept higher than the temperature of the constant temperature bath 11 by the function of the temperature control device 7 and the heater 6. On the other hand, as previously noted, the temperature in the periphery of the sample 1 is approximately equal to the temperature of the constant temperature circulator bath 4. If this temperature is taken to be Td, the relative humidity RH produced by the atmosphere at the vapor pressure P(Tw) introduced in the periphery of the sample is expressed in the following equation;

RH(%)={P(Tw)/P(Td)}×100  (1)

[0021] Accordingly, to set the sample temperature at Td and the sample humidity at RH(%), the temperature Tw, the temperature for the constant temperature bath 11, is given by the next equation after solving the equation (1);Namely,

Tw=P-1[RH(%)×P(Td)/100]   (2)

[0022] Here, P-1 is the inverse function of P, which converts the saturated vapor pressure into the corresponding temperature. Namely, T=P-1[P(T)].

[0023] At the constant temperature bath temperature setting device 26, Tw is calculated based on the equation (2), and the temperature of the constant temperature bath 11, that is, the temperature of the glass bottle 8, is controlled while setting Tw as its target.

[0024] At this point, the relative humidity in the periphery of the sample 1 is detected by the humidity sensor 19 periodically, and converted into the humidity signal at the humidity measuring circuit 20.

[0025] The weight of the sample 1 may change with the change of the temperature or humidity. The change in the weight of the sample is detected at the balance unit 15 after passing through balance beam 14, and converted into the thermogravimetry(TG) signal at the TG measuring circuit 21.

[0026] Signals corresponding to the temperature, TG, the humidity measured at the TG measuring circuit 21 and at the humidity measuring circuit 20, are transmitted to the signal stability determination circuit 22, where the stability of each signal and the difference of the temperature and humidity signal from the target value is evaluated.

[0027] At the signal stability determination circuit 22, when temperature and humidity signals fulfill the standard condition for the preset rate of change, for example, 0.2□ per minute or under and 1% per minute or under, the difference between the measured value and the set target value of the temperature and humidity is evaluated. When the difference in temperature is 0.20 or more, or the difference in humidity is 1% or more, a signal is transmitted to the temperature and humidity target signal generating device 24 indicating the correction required to Td or Tw and depending at the quantity and direction of the difference. When the measured temperature and humidity signals fulfill the standard condition of the prescribed rate of change, and, the difference from the preset target value is within the standard, the stability of TG signal is evaluated.

[0028] Even when the temperature and humidity conditions in the periphery of the sample become stable, the TG signal does not always turn out to be stable. Depending especially on the property or the shape of the sample, the time required to reach humidity equilibrium for the sample and for the atmosphere differ. However, whether or not the humidity equilibrium is reached can be directly evaluated according the stability of the physical property signal obtained by continuous measurement of the physical property of the sample. Namely, the stability of the TG signal can be evaluated whether or not the condition specifying the rate of change standard, for example, 0.01 mg per minute or under, is fulfilled. When this TG stable condition is fulfilled, a trigger signal to update the humidity target value in stepwise manner is transmitted from the signal stability determination circuit 22 to the temperature and humidity target signal generating device 24.

[0029] As a result, the target temperature of the constant temperature bath 11 is updated via the constant temperature bath temperature setting device 26. Accordingly, the temperature of the glass bottle 8 changes. Finally, the relative humidity of the atmosphere in the periphery of the sample changes, and the change is measured at the humidity measuring circuit 20 after passing through the humidity sensor 19.

[0030] Thus, one step in a series of humidity control steps is completed. The same humidity scanning is repeated in a stepwise manner until the prescribed humidity control range is covered, after which the measurement will be completed.

[0031] In this embodiment, the humidity is controlled by introducing the saturated vapor corresponding to the wet-bulb temperature into the sample chamber in which the temperature is controlled corresponding to the dry-bulb temperature of the wet- and dry-bulb type hygrometer. In other apparatus in which the desired relative humidity is obtained by mixing dry nitrogen and saturated vapor at a certain ratio, there are the disadvantages that it is expensive because of its complicated device composition and also that it lacks versatility as it cannot be used with thermal analysis apparatus. However, a series of humidity control steps can be performed by changing the mixture ratio in stepwise manner.

[0032] Referring to the types of physical property signal, Differential Scanning Calorimetry (DSC) signal, sample length (TMA)signal, dynamic viscoelasticity signal, for example, storage modulus or tangent delta, can be applied to the typical device used in the thermal analysis apparatus.

[0033] FIG.2 is a time chart schematically showing the time change of the thermogravimetry(TG) signal, relative humidity (RH) signal, and the temperature (Td) signal of the sample 1 during the performance of a series of humidity step control.

[0034] As mentioned above, according to the present invention, temperature and humidity of the sample are measured, and the difference from a set target value is continuously corrected. Thus the temperature and humidity conditions of the sample can be controlled as set by a target value. And humidity is scanned based on the stability of a physical property signal, which reflects changes in a physical property of the sample, in a stepwise manner without estimating the time required to reach humidity equilibrium in advance, which differs depending on properties or shape of the sample. Thus humidity dependency of a physical property of the sample can be accurately analyzed. Accordingly, the redundancy of the constant speed humidity scanning method usually required the measurement data corresponding to the uncertain portion of the time required to reach the humidity equilibrium is solved. Therefore the analysis time can be kept to a minimum. Also, the occurrence of error due to hysteresis in the constant speed humidity scanning method caused by the difference of the humidity scanning direction based on an underestimate of the humidity equilibrium time can be solved. Moreover, the apparatus of the present invention is composed by replacing the heating furnace of the thermal analysis apparatus with a simple humidity control system mainly comprising two constant temperature baths. This means it has a versatility, and can be used as a thermal analysis apparatus by putting back the heating furnace. The instrument capable of automatic humidity step control measurement at low cost, and is suitable for evaluation of hygroscopicity, for example, chemicals.

[0035] Also, by controlling the humidity of the sample chamber, even in situation where the sample has previously reached its equilibrium state at a certain humidity and temporarily deviates from its equilibrium, confirmation that it has returned to its equilibrium state by the measurement start can be performed with the trigger signal. Therefore, evaluation of the temperature dependency of the changes in the weight of the sample and the thermal change can be started without deviating from its equilibrium state. Thus the thermal characteristics of the sample absorbed moisture can be evaluated accurately. 

What is claimed:
 1. An automatic humidity step control thermal analysis apparatus comprising; a hot water chamber for generating saturated water vapor at the desired temperature capable of temperature step control, a sample chamber for holding sample and capable of changing the temperature and humidity of the stored sample and its inside, a heat insulation pipe for transmitting water vapor from the hot water chamber to the sample chamber while preventing dew condensation, a detector for measuring physical properties of the sample, a signal stability determination circuit for generating a trigger signal in the event that the rate of change of physical property signal from the detector drops below a specified reference value, wherein the humidity dependency of changes in the physical properties of a sample is evaluated by changing the humidity inside the sample chamber in a stepwise manner according to the ratio of the respective saturated vapor pressures at the temperature of the hot water chamber and at the temperature of the sample chamber, made possible by adjusting the water temperature within the hot water chamber in a stepwise manner corresponding to the trigger signal.
 2. An automatic humidity step control thermal analysis apparatus of claim 1, comprising; a physical property signal measured by the detector expressing the mass of sample.
 3. An automatic humidity step control thermal analysis apparatus of claim 1, wherein a physical property signal measured by the detector expresses the difference in temperature between the sample and a reference material.
 4. An automatic humidity step control thermal analysis apparatus of claim 1, wherein in the event that a sample is determined to have reached a state of equilibrium at a certain humidity by a trigger signal of the signal stability determination circuit, the temperature of the sample is changed without making deviation from the equilibrium state to evaluate the temperature dependency of the sample.
 5. An automatic humidity step control thermal analysis apparatus of claim 4, wherein a physical property signal measured by a detector expresses the mass of the sample.
 6. An automatic humidity step control thermal analysis apparatus of claim 4, wherein a physical property signal measured by the detector expresses the difference in temperature between the sample and a reference material. 